In prior overhead projectors of comparable design, a stage for a slide or transparency to be imaged, means including a source of light and a condenser for illuminating the slide or transparency, and a projection lens system to form the image have been mostly installed parallel to each other. The axis of the light from the illumination means passes through the respective centers of the stage and the projection lens system at right angles, and is directed toward a screen by an inclined mirror above the lens system. In case the screen is installed vertically, however, the image produced thereupon is inevitably distorted into the shape of a trapezoid, tapering toward the lower end. This distortion has so far been compensated for by mounting the screen out of the perpendicular.
Such an inclined screen does not provide a favorable view of the image being projected thereupon, especially to an audience of many persons. This is a critical drawback to the prior overhead projectors, as these are principally designed for such large audiences in classrooms, auditoriums, assembly rooms, and other halls or rooms of various establishments. Particularly in rooms or theaters with amphitheatrical seat arrangement, it must be a toil indeed for those in the rearward seats to watch the inclined screen for an extended length of time.
Accordingly, it is an object of the present invention to provide a novel and improved system of overhead projection whereby an undistorted image of a transparency of slide (hereinafter simply referred to as a "transparency" to mean any such picture or like material adapted for projection in an overhead projector) can be formed on a vertical surface or screen (hereinafter simply referred to as a "screen").